A DC/DC switching power supply system feeds back its output voltage to be compared with a reference voltage to generate an error signal, and the error signal is further compared with an oscillating signal such as ramp signal and triangle signal, to determine a duty that is used to regulate the output voltage. For clearer illustration, FIG. 1 shows the corresponding part of a DC/DC switching power supply system, in which a feedback signal FB is derived from the output voltage of the switching power supply, an error amplifier 10 compares the feedback signal FB with a reference signal Vref to produce an error signal comp, and typically a compensation circuit 14 is connected to the output node of the error amplifier 10 to compensate the error signal comp. A comparator 12 compares the error signal comp with an oscillating signal SLOP which is a ramp or triangle signal, and the output signal of the comparator 12 includes the duty information for switching the power switch of the switching power supply. FIG. 2 is a diagram showing an operation region of the error signal comp. For better regulation, the minimum level V1 of the error signal comp. for normal mode operation is usually a non-zero voltage. If the error signal comp is higher than the minimum voltage V1, the switching power supply operates in a Pulse Width Modulation (PWM) mode, and the error signal comp will be compared with the oscillating signal SLOP to determine the duty. If the error signal comp is lower than the minimum voltage V1 for a time interval, the switching power supply will enter a Pulse Frequency Modulation (PFM) mode or sleep mode. In the sleep mode, since the load current is so low that the output voltage of the switching power supply decreases very slowly, the switch cycle of the power switch will be much longer, namely the charge frequency for the output capacitor decreases dramatically. Consequently, the error signal comp continuously goes down and likely becomes much lower than the minimum voltage V1. In this situation, if the switching power supply suffers more loading suddenly, the error signal comp would be difficult to rapidly recover to be higher than the minimum voltage V1, and thereby the switching power supply could not be rapidly waked up from the sleep mode.